Carbon-isotope evidence reported from 3.7 Ga Isua sediments

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Minik Rosing reported graphite microparticles in more than 3,700 million-year-old seafloor turbidites from Isua in West Greenland whose carbon-13 content is depleted to a degree characteristic of biological carbon fixation. Organisms preferentially take up the lighter isotope, so biological carbon is isotopically light and the residual inorganic pool is correspondingly heavy — a fractionation that is easy to measure and hard to produce abiotically at the same magnitude. The claim is important because it does not depend on any structure being biological in shape, which is the weak point of every stromatolite and microfossil claim of comparable age. Its own weak point is that certain abiotic reactions, including some hydrothermal and metamorphic processes, can produce isotopically light carbon, so the argument turns on how large a fractionation is achievable without life.

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